In valves of this type the effective surface of the control piston must be greater than the effective surface of the sealing element which in one switch position breaks the connection between the consuming device which is supplied with pressure medium and the appertaining connection for removal of the pressure medium. Only when the control piston is supplied with pressure medium can it move the piston rod into the other switch position.
However, this necessary dimensioning of the control piston results in a problem if an instantaneous excess pressure occurs as a result of external influences (for instance simultaneous venting of air from a plurality of valves connected to a common exhaust air pipe) in the housing chamber which connects the consuming device from which air has just been vented to the appertaining connection for removal of the pressure medium. If this excess pressure were to act on the inner end face of the adjacent control piston this would cause unintentional actuation of the valve. i.e. incorrect actuation.
In order to avoid this disadvantage, in the previously known valves of the type set out in the introduction two sealing and pressure-equalising pistons, each bearing sealing elements which co-operate with a bushing pressed into the valve housing, are arranged between the control piston and the chambers of the housing provided with the connections for removal of the pressure medium.
These two additional sealing or pressure-equalising pistons have in particular the undesirable effect of increasing the length of the valve. There are further disadvantages in the increase in the number of individual parts of the valve, the complication of the assembly and the shortening of the working life.